1. Field of the Invention
This invention relates to reduction in carbon oxides products in oxidative pyrolysis of halogenated methanes in gas phase halogen-catalyzed oxidative-pyrolytic, non-flame, conversion of methane to higher molecular weight hydrocarbons carried out in the presence of oxygen-containing gas which is primarily introduced in a latter portion of the process sequence to reduce the oxygen requirement for effective suppression of the formation of carbonaceous deposits and carbon monoxide.
2. Description of the Prior Art
Natural gas contains varying quantities of methane, typically about 75 weight percent, and thus constitutes an important raw material for the synthesis of higher molecular weight hydrocarbons. Various processes are known for the conversion of methane into acetylene, ethylene and hydrogen using high temperature pyrolysis. However, thermal decomposition of methane results in solid carbonaceous deposits which reduces the yields for desired hydrocarbon products. The reduction of formation of carbonaceous deposits, such as tars, solid carbon and soot, while maintaining high yield for the desired hydrocarbon products, is obtained by oxidative pyrolysis of halogenated methanes in the gas phase and under non-flame conditions in the presence of oxygen as described in U.S. Pat. No. 4,714,796, which is incorporated herein by reference in its entirety. The 4,714,796 patent teaches that as halogenated methane conversion is increased, the molar ratio of ethylene to acetylene in the product decreases at long reaction times. However, since the feed initially did not contain any ethylene its concentration must have earlier increased in the process. Under the conditions of Example 1 of that patent, ethylene to acetylene molar ratio changed from less than about 2 to less than about 1 along the reactor length. Similar molar ratios of ethylene to acetylene product are reported in "Conversion of CH.sub.4 into C.sub.2 H.sub.2 and C.sub.2 H.sub.4 by the Chlorine-Catalyzed Oxidative-Pyrolysis (CCOP) Process 1. Oxidative Pyrolysis of CH.sub.3 Cl", A. Granada, S. B. Karra, and S. M. Senkan, Ind. Eng. Chem. Res., Vol. 26, No. 9, pgs. 1901-1905 (1987) and in "Converting Methane by Chlorine-Catalyzed Oxidative Pyrolysis", S. M. Senkan, Chemical Engineering Progress, pgs. 58-61, December 1987. The 4,714,796 patent teaches that carrier gases or mixtures may be used to reduce the concentration of the active reactants and that inert carrier gas may be used, or other gases which do not contain interfering compounds may be used. The 4,714,796 patent teaches that methane or any gaseous source of methane may be used as a carrier gas. Although the 4,714,796 patent teaches the production of ethylene and acetylene from methane by oxidative pyrolysis of halogenated methanes, the carbon oxides also form in the process. Since ethylene is presently a much more valuable product, development of a process providing lower carbon oxides production while maintaining low levels of carbonaceous deposits is desirable.
In a related investigation, Weissman and Benson studied the kinetics of high temperature non-oxidative pyrolysis of methyl chloride under non-flame conditions reporting formation of significant amounts of carbon. Weissman and Benson also teach that increase in methane presence causes significant increase in carbon formation. Weissman, M. and Benson, S. W. "Pyrolysis of Methyl Chloride, a Pathway in the Chlorine-Catalyzed Polymerization of Methane", In. J. Chem. Kinetics, Vol. 16, p. 307-333 (1984).